The best way, from my opinion is just unban him without any explanations and warnings and consider it not as a faliure of administration and dangerous achievement of users community which can lead to something extremely terrible which doesn't come to my poor imagination. And start following your own rules - no permabanning before temp bans. Ideally no permabanning at all. If someone was tempobanned 3 times, and still returns and makes new mods
... yes, I would give him 4th tempoban. As exclusion. And then 5th, etc. If he is such a maniac that he returns each time and makes new mods - Shrapnel should be just proud of it. It means he is a very loyal citizen of Shrapnel world. It's pretty cool. And sometimes people with lack of communication skills, if they have weight in community, they DESERVED it. And, having weight in community DE-FACTO in comparison with many admin's DE JURE means a lot. We are young people, we like democracy and fun. If we were serous old farts in a costumes who think too much about rules we wouldn't play computer games at all.
Please make this world, which you create for us (I hope so!) a pleasant place for US, even if it in this case will be less pleasant for you, those who regulate. You are earning money for being in this world. And we are - not. We are here only because of pleasure and only for it.
If you say you banned Sombre not for yourself, but for community...I didn't hear much voices from unpaid community users who would support you. That's a reality.